Add the "doas" alias to eshell.

Add the "doas" alias to eshell.

  * lisp/eshell/em-tramp.el (eshell/doas): new function.
  (eshell--method-wrap-directory): new function.
  (eshell/sudo): accept '-s'/'--shell' for interactive use.
  * test/lisp/eshell/em-tramp-tests.el
  (em-tramp-test/sudo-shell) (em-tramp-test/sudo-user-shell)
  (em-tramp-test/doas-basic) (em-tramp-test/doas-user)
  (em-tramp-test/doas-shell) (em-tramp-test/doas-user-shell): new
  tests.
  * etc/NEWS: mention new 'doas' eshell command.
  * doc/misc/eshell.texi: add 'doas' command documentation.